| | | # Elevator Power.
Asked Feb 20, 2007, 08:18 AM
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** 7 Answers** | A 550 kg elevator starts from rest. It moves upward for 5.00 s with constant acceleration until it reaches its cruising speed, 1.75 m/s.
(a) What is the average power of the elevator motor during this period?
W
(b) How does this power compare with the motor power when the elevator moves at its cruising speed?
Pcruising = W
The formula for average power is avg. power = work/change in time
However, I am not sure how to find the work in the problem since no force or angle is given... Is gravity the only force doing work and is the angle 90? |
7 Answers
| Uber Member | |
Feb 20, 2007, 09:35 AM
| | The angle is 0 (you don't need to worry about it, because it's a 1d problem), the motor is doing work against the force of gravity. | | | New Member | |
Feb 20, 2007, 02:12 PM
| | Ok so the F in the problem is 9.8m/s^2. I would use cos0. However, the change in distance is unknown. Using the formula d=time * speed, I found it to be 8.75m. So, now would I use the work formula with the numbers (9.8m/s^2)(8.75m)(cos0). Which equals 85.57J. After I find that I would divide work by time (5secs) to find average power which is 17.15 W... just my thinking? | | | Uber Member | |
Feb 20, 2007, 02:33 PM
| | F = ma
your distance is wrong because it's accelerating, you need to use suvat equations | | | New Member | |
Feb 20, 2007, 03:01 PM
| | The force in this problem would be the mass of the elevator time the acceleration which is 1.75m/s correct. So f in the problem would be 962.5N. For distance I mult. The speed times the time. That is the only way I know to find distance. | | | Uber Member | |
Feb 20, 2007, 03:13 PM
| | acceleration is the acceleration of gravity!!
you need to use s = ut + 1/2at^2 to find distance traveled, because it is under constant acceleration.
I have to hit the sack now, but I will carry on tomorrow. | | | New Member | |
Feb 20, 2007, 04:22 PM
| | One more try:
f=mass*acceleration
f=550*9.8=5390N
change in distance=122.5m
cosign of angle 0degrees
w=f*change in d*cosign0
w=5390*122.5*cosign0
?? Correct? | | | Uber Member | |
Feb 20, 2007, 11:31 PM
| | Oh no, your acceleration for s = ut + 1/2 at^2 is 1.75/5 | | Thread Tools | Search this Thread | | | ##
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